1. Field of the Invention
The present invention relates to a substrate for an information recording medium, to a method of producing the substrate, and to an information recording medium comprising the substrate.
2. Description of the Related Art
As a substrate for an information recording medium, attention is recently directed to a glass substrate and a ceramic substrate which are excellent in various properties as compared with other substrates. For example, an aluminum substrate has been widely used as a substrate for a magnetic disk. With a demand for reduction in size and thickness of the magnetic disk and for a low flying height of a magnetic head, the glass substrate and the ceramic substrate become more and more widely used. This is because the glass substrate and the ceramic substrate can be readily reduced in size and thickness and has an excellent flatness so that the flying height of the magnetic head can easily be lowered as compared with the aluminum substrate.
In the case of using the glass substrates for information recording media, their surface is often subjected to chemical strengthening treatment to increase their strength in order to improve their impact resistance and vibration resistance to thereby prevent them from being broken by shock or vibration.
For the chemical strengthening treatment, for example, an ion-exchanging method is much employed in which the alkali ions in glass are substituted with other alkali ions having a larger ionic radius to thereby increase the volume of the ion-exchanged site in the glass processed. In that method, the increase in the volume of the ion-exchanged site gives strong compression stress to the surface layer of the glass processed whereby the glass surface is strengthened. In the ion-exchanging method for chemical strengthening, the glass substrates to be processed must contain alkali ions because of the principle of the method itself. In some types of glass for substrates for information recording media, containing alkali ions are known which have a predetermined strength even without being subjected to chemical strengthening treatment, for example, high-valence glass.
On the other hand, for ceramic substrates for information recording media, such as crystal glass substrates, in general, they do not require chemical strengthening treatment as their impact resistance and vibration resistance are generally increased through crystallization.
Using glass or ceramic substrates in information recording media is often problematic in that the alkalis existing in the glass or ceramic substrates are extracted out, and it is desirable to minimize the alkali extract from those substrates. Even the ion-exchanged glass substrates give the trouble of alkali extract from them.
As mentioned above, the problem with glass or ceramic substrates for information recording media is the alkali extract from them, but, at present, no high-level technique of preventing the alkali extract is known.